Karl Hoglund, the police chief of Sebring, Fla., choked back tears as he described a shooting in a SunTrust Bank in Florida. Those killed are “victims of a senseless crime,” he said.CreditCreditEve Edelheit for The New York Times

The gunman who burst into a SunTrust Bank in Florida on Wednesday made the five women he found inside lie facedown on the floor before he shot them each in the back of the head, killing them, according to an affidavit released on Thursday. And when he was done, he called the police to tell them what he had done.

On Thursday, the police released the identities of three of the five victims — four female bank employees and one female customer — and said for the first time that a sixth person inside the bank in Sebring, Fla., escaped when he heard the shooting begin around 12:30 p.m.

The suspect, Zephen A. Xaver, 21, was arrested on Wednesday and charged with five counts of first-degree premeditated murder. Nathaniel Heitkamp, a friend who said in an interview that he met Mr. Xaver five years ago at a mental health facility in Indiana, said, “He had an obsession with violence.”

Chief Karl Hoglund of the Sebring Police Department identified the customer who was killed as Cynthia Lee Watson, 65, and one of the employees as Marisol Lopez, 55.

At a news conference later in the day, a third victim, Ana Piñon-Williams, 37, was identified by her brother-in-law, Tim Williams. He said she was a mother of seven who started working at the bank recently.

“We do not know what was going on in the mind of the individual who committed this atrocious act, but we do know he was influenced by the darkness in this world,” Mr. Williams said.

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Ana Piñon-WilliamsCreditvia Sebring Police Department

The police said they also did not understand why Mr. Xaver, wearing a T-shirt with an image of four scythe-wielding grim reapers on horseback, carried out the methodical killings in the small city, about 80 miles south of Orlando.

Chief Hoglund said on Thursday that Mr. Xaver did not know any of the victims and had no known connection to the SunTrust branch. The chief also said there were no signs that he had intended to rob the bank or do anything there other than shoot people.

“We believe it was a random act,” Chief Hoglund said. “We are still trying to establish what has occurred, the gravity and nature of why it occurred, and try to put it in a perspective that we can understand.”

When officers arrived at the bank on Wednesday, they found a harrowing scene, according to the police affidavit released on Thursday.

Mr. Xaver, who remained on the phone with police, was barricaded inside the bank and said he had a nine-millimeter handgun and was wearing a bulletproof vest. What followed was a standoff that ended when an armored police vehicle rammed into the bank doors, shattering their glass, video footage shows.

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The police entered the SunTrust branch on Wednesday afternoon after ramming an armored police vehicle through the bank’s door.CreditEve Edelheit for The New York Times

Inside, the police found five women lying in the lobby with gunshot wounds to the backs of their heads and upper torsos, spent shell casings littered on the floor all around them. They found Mr. Xaver hiding in an office in the back of the bank, the affidavit said.

The police initially said on Wednesday that the five victims were the only people in the bank at the time of the attack, but Chief Hoglund said on Thursday that a sixth person had been on the premises when Mr. Xaver opened fire.

“There was another bank employee in the building at the time the incident began,” the chief said in an email. “The employee was in a back break room and heard the shots and ran out a back door and contacted law enforcement.”

Relatives of the two other victims asked that their names be withheld from the public, Chief Hoglund said on Thursday, adding that he would honor those requests under a new crime victims’ law in Florida known as Marsy’s Law.

The shooting on Wednesday was the latest of several high-profile deadly attacks in Florida in the last year. The authorities did not say on Thursday how Mr. Xaver had obtained a gun.

At a court hearing on Thursday morning, Mr. Xaver, wearing a black-and-white jumpsuit, stood before Judge Anthony Ritenour and responded, “Yes, sir,” when asked whether he had no income or assets. The judge appointed a public defender to represent Mr. Xaver, who was being held at Highlands County Jail in Sebring, and ordered him held without bond.

Mr. Xaver lived in Sebring but had spent most of his life in Plymouth, Ind., a town about 23 miles south of South Bend, Ind., according to Mr. Heitkamp. Mr. Xaver had recently trained to be a correctional officer at Avon Park Correctional Institution, a prison about 20 miles north of Sebring, said Patrick Manderfield, a spokesman for the Florida Department of Corrections.

Mr. Heitkamp said he had met Mr. Xaver when they were both teenagers and patients at Michiana Behavioral Health in Plymouth. A representative at Universal Health Services, the company that operates the center, did not immediately respond to a request for comment on Thursday.

Over the years, Mr. Xaver complained about being bullied at school and disliked by his family, said Mr. Heitkamp, who now works in the service industry. When Mr. Xaver got upset, Mr. Heitkamp said, he openly talked about a desire to hurt people and how he had access to guns.

“This man did not hide it,” Mr. Heitkamp said.

Gracelynn Williams, 20, said she became close friends with Mr. Xaver at the same facility in Plymouth several years ago.

Reached through Facebook, Mrs. Williams said Mr. Xaver had struggled with suicidal and homicidal thoughts and had a strained relationship with his parents, especially with his father.

“He mostly spoke about wanting to hurt himself,” she said.

“Society made him a monster,’’ she said. “Unfortunately, nobody listened to him and five lives have been lost. It’s devastating.”

Correction:Jan. 24, 2019

An earlier version of this article transposed the identities of two of the people killed at a SunTrust Bank branch. Cynthia Watson was a customer and Marisol Lopez an employee.

Kitty Bennett and Doris Burke contributed research.

A version of this article appears in print on , on Page A17 of the New York edition with the headline: ‘A Random Act’: Details Emerge in Bank Shooting as Victims Are Identified. Order Reprints | Today’s Paper | Subscribe